Convert BMP to JPG

This is the property that sends people here. A BMP writes three bytes for every pixel with no compression stage in between, so its size is decided by the dimensions alone: 6.2 MB for 1920 by 1080, 26 MB for an A4 page scanned at 300 dpi, 36 MB for a 4000 by 3000 photograph. A blank white image of those dimensions is exactly as large as a detailed one.

JPEG works the other way round. It transforms blocks of pixels into frequency coefficients and spends bits only where there is something to describe, so a flat diagram collapses to a few tens of kilobytes while a noisy photograph stays substantial. That is the whole of the difference, and it is why nobody can tell you in advance how small your particular file will get. It is also why the answer people want — a ratio — does not exist for this pair. The only honest procedure is to convert the file and look at the number, which takes a second here because the work happens on your own machine rather than after an upload.

Detail the eye is poor at noticing: fine variation in colour more aggressively than fine variation in brightness, and the highest-frequency content within each block. On a photograph at a sensible setting this is genuinely invisible at normal viewing size, which is why JPEG has been the default photographic format since 1992.

It is not free anywhere else. Hard edges are precisely the high-frequency content the transform throws away first, so text, screenshots, line art and a logo on flat colour pick up a visible haze around every boundary. The material decides whether this conversion is a good idea, and the material in a BMP is very often a screen capture. The tell is easy to spot once you know it: zoom to 100 per cent and look at the pixels immediately around a letter or a hard border. A faint grey ripple that was not in the bitmap means the encoder is spending its budget on edges it was never designed to preserve.

The encoder here defaults to 82 on the usual 1 to 100 scale. That is a deliberate choice rather than an arbitrary one: it sits just above the point where artefacts start to be visible on ordinary photographs, and it is the setting that makes the trade look free.

The savings live below it. Going from 82 to 70 makes a real difference to the byte count and is still perfectly acceptable for something being viewed on a phone; going from 82 to 95 costs a great deal and buys almost nothing a person will see. The scale is not linear and it is not a percentage of anything — it is the encoder’s own dial, and the same number in WebP or AVIF means a different amount of loss. If the file still will not fit after all that, the lever to reach for is the dimensions rather than the quality number, and the next section but one is about why.

JPEG has no alpha channel — not a reduced one, none — so any see-through pixel has to become opaque before it can be encoded. Where the source is one of the 32-bit BMP variants that do carry alpha, the image is flattened onto a background first.

That background is white, and this page does not ask you which colour it should be. It is a real gap rather than a design decision: the pages that convert a PNG into an opaque format do offer the choice, and this one inherits a default. If the fill colour matters to you, flatten the image deliberately in an editor before converting rather than discovering the result afterwards.

The BMP you started with holds every pixel exactly. The JPEG does not, and there is no setting, no tool and no later conversion that recovers what the encoder decided to leave out. Converting the JPEG back to BMP produces a large file containing the losses rather than the original.

So keep the BMP if it is the only copy. The rule that follows is worth carrying beyond this page: edit and archive in the lossless file, export to JPEG at the moment of sending, and never edit the JPEG and save it again — each round through the encoder degrades it further, which is the generation loss the format is known for.

If the BMP is a screen capture, a UI mock-up, a chart, a scanned line drawing or anything else made of flat colour and hard edges, this is the wrong target. JPEG will visibly degrade the type and, for that material, frequently produces a larger file than a lossless format would.

PNG is the answer there and it is one click away. That is not a small distinction: BMP files come disproportionately from Paint, from capture utilities and from scanner drivers, so the flat-material case is not the exception on this page — it is close to half of what arrives. A 960 by 640 screen capture that is 1.84 MB as a bitmap came to 468 KB as a PNG in a measurement on this site, with every pixel exactly as it was. A JPEG of the same image would not reliably beat that number and would certainly not match the result.

JPEG is one of the richest metadata carriers here — EXIF, XMP, IPTC, an ICC profile and GPS coordinates all have a defined place in the file, which is why photographs from a phone carry so much history around with them.

A BMP has none of it. The registry records no metadata containers for the format at all, and this pipeline hands the encoder a grid of pixel values and nothing else, so the JPEG comes out with no capture time, no device name, no software field and no location. For once that makes this the privacy-preserving direction by default rather than by effort — there is nothing to strip, because there was never anything to strip.

The dimensions are the strongest lever available and the one people reach for last. Halving the width of an image quarters the pixel count, and the JPEG shrinks by something close to the same factor while looking identical anywhere it is displayed smaller than full size.

A 4000 by 3000 photograph reduced to 2000 by 1500 is still larger than any phone screen and larger than the printable area of most documents. Set the maximum width before converting and it is applied in the same canvas pass as the rest of the work, so the picture is resampled once rather than twice and no fringe is introduced along the way. Combining a modest reduction with quality 75 will get almost any bitmap under almost any limit, and in that order — reduce first, then lower the quality only if it is still too large.

Each file may be up to 100 MB, which is about 33 megapixels of uncompressed bitmap. Drop a folder and every file converts in turn, keeps its name with the extension changed, and the set comes back as a single ZIP with no queue and no daily allowance.

Because the outputs are small, batches here are limited by the sources rather than by the results — the opposite of converting to BMP. A folder of 300 scanner outputs is unremarkable; a single 200 MB bitmap is the case that will not go through.

Rarely because nothing can read it. Every browser displays a BMP, and the format is documented and ancient enough that decoders are everywhere. What refuses it is the layer above: attachment limits, upload forms that accept a short list of extensions, and content systems that will not store an image measured in tens of megabytes.

JPEG passes all of those for the same reason: it is what those systems were written to expect. It has been the default photographic format since 1992, it is specified in ITU-T T.81, and there is no device or service left that cannot display one. That makes this conversion an act of compatibility as much as of compression, and it is the reason to do it at the boundary — at the moment of sending — rather than to the copy you keep.